Displays, and in particular, matrix driven displays are particularly although not exclusively useful for television receivers and visual display units for computers, especially although not exclusively portable computers, personal organisers, communications equipment, and the like.
Referring first to FIG. 1, a magnetic matrix display such as that disclosed in UK Patent Application 2304981 comprises: a first glass plate 10 carrying a cathode 20 and a second glass plate 90 carrying a coating of sequentially arranged red, green and blue phosphor stripes 80 facing the cathode 20. A permanent magnet 60 is disposed between glass plates 90 and 10. The magnet is perforated by a two dimension matrix of perforations or “pixel wells” 70. An array of anodes 50 are formed on the surface of the magnet 60 facing the phosphors 80. There is a pair of anodes 50 associated with each column of the matrix of pixel wells 70. The anodes of each pair extend along opposite sides of the corresponding column of pixel wells 70. A control grid 40 is formed on the surface of the magnet 60 facing the cathode 20. The control grid 40 comprises a first group of parallel control grid conductors extending across the magnet surface in a column direction and a second group of parallel control grid conductors extending across the magnet surface in a row direction so that each pixel well 70 is situated at the intersection of a different combination of a row grid conductor and a column grid conductor. In operation, electrons are released from the cathode and attracted towards control grid 40. Control grid 40 provides a row/column matrix addressing mechanism for selectively admitting electrons to each pixel well 70. Electrons pass through grid 40 into an addressed pixel well 70. In each pixel well 70, there is an intense magnetic field. The pair of anodes 50 at the top of pixel well 70 accelerate the electrons through pixel well 70 and provide selective sideways deflection of the emerging electron beam 30. Electron beam 30 is then accelerated towards a higher voltage anode formed on glass plate 90 to produce a high velocity electron beam 30 having sufficient energy to penetrate the anode and reach the underlying phosphors 80 resulting in light output.
The control grid 40 provides a row/column matrix addressing mechanism for selectively admitting electrons to each pixel well 70, which is used to address individual pixels a line at a time. Inevitably there will be small luminance non-uniformities from pixel to pixel. These fall into at least three categories of non-uniformities. The three categories are described individually here as:    1. Luminance changes caused by temperature changes during warm up after initial display switch on. This is particularly a problem in a display using a thermionic cathode as considerable heat is generated in the display by the thermionic cathode.    2. “Washboard” luminance errors typical of a device using thermionic filament cathodes, or a photocathode with backlighting.
F G Oess, “The uniform remote virtual cathode system”, SID digest 1994”, Feb. 23, 1996, discloses that in the design of thermionic area cathodes for flat CRTs it is necessary to create a flat plane of electrons remote from the electron sources. The electron sources are a number of long hot cathode filaments. FIG. 2 shows the electrons associated with a single cathode filament 202 of a multiple hot filament cathode. The filament can be considered in one plane as a point source of electrons and the electron distribution from that one filament is not flat, but is curved with respect to the magnet 60. The cathode extractor grid is shown at 204 and the magnet of the magnetic matrix display at 60. A wavefront of electrons which have been extracted from the single cathode filament 202 and which form a virtual cathode is shown at 206.
More than one of the filaments shown in FIG. 2 are always used in a practical area cathode. FIG. 3 shows the wavefront 306 of electrons associated with the multiple cathode filaments (202a through 202f) which make up a thermionic area cathode. The curved distribution of electrons from each of the filaments 202a through 202f causes the overall distribution of electrons to be the sum of a number of curves, tending to have a “wavy” structure. This “washboard” problem may be further exacerbated by position tolerance variations in the filament wires.
It is theoretically possible to use one cathode filament at each pixel column or row spacing, when the cathode plane modulation will be exactly coincident with the pixels and hence there will be no brightness modulation observable on the screen. However, this would result in an excessive number of filaments and would be impossible simply because the power requirement would be too large (each filament has to be heated to 750° C.).
U.S. Pat. No. 3,769,540 discloses the use of extra extraction electrodes, for example an extraction “cage” round each filament, but this increases mechanical cost and complexity, and is subject to manufacturing tolerances.
European Patent Application 0 539 679 A discloses that the extraction grid can be shaped into a “counter” curve, but again this is subject to mechanical tolerances.
Extra grids and a repeller can also be added in order to reduce the problem of “washboard” luminance errors, but cost, complexity and tolerances are a problem with such additions.    3. Production tolerances in parameters, such as, for example, magnet hole diameter, grid hole diameter, spacing of electrodes and the like.
Nonomura et al, “A 40″ Matrix Driven High Definition Flat-Panel CRT”, SID Digest 1989, pp 106–109 describes how variations in cathode emission are compensated in a Colour Flat Panel product, but corrections are applied for cut-off only, to equalise emissions between individual emission wires, and are not applied at an individual pixel level. Neither Nonomura, nor other workers in this field disclose both cut-off and gain tolerance variation correction at the pixel level.
The electrostatic operation of a magnetic matrix display, in particular the key CRT parameters of cut-off voltage and beam current modulation, are controlled by the following equations:Vc=(0.036×VD)+(0.6×Vg2)×P  (1)Ib=0.124×Vg21.4×√{square root over ((VD+0.011VS)×P)}  (2)Where:                     P        =                              79.06            ×                          D              3                                                          d                              c                -                g1                                            3                /                2                                      ×                          d                              D                -                g2                                      ×                          [                              15                +                                  d                  g1–g2                                            ]                                                          (        3        )            Where:                Vc=Cut-off voltage        VD=Deflection anode voltage        Vg2=G2 voltage (row grid conductor voltage)        P is a constant dependent on the dimensions of the display        Ib=Beam current        VS=Screen voltage        D=Aperture diameter        dc-g1 is the dimension from the cathode to G1 (column grid conductor)        dD-g2 is the dimension from the deflection anode to G2 (row grid conductor)        dg1-g2 is the dimension from G1 (column grid conductor) to G2 (row grid conductor)        
In the equation for the constant P, the dimensions (d) will vary with manufacturing tolerance. Because P appears in both the cut-off and beam modulation equations then these variations will affect both cut-off and beam modulation.